Seismic Performance of Square Steel Tube Concrete Columns Under Cyclic Loading

Literature Overview

The paper by Nie Ruifeng, Xu Peizhen, and Yan Yu (Journal of Tongji University, Natural Science Edition, 2012, Vol. 40, No. 11, pp. 1596-1602) presents a comprehensive experimental and numerical study on the seismic performance of square steel tube concrete (STC) columns. Six full-scale specimens were tested under low-cycle reversed loading, and the results were validated using Abaqus finite element simulation. The study investigates the influence of key parameters — steel ratio, axial compression ratio, and slenderness ratio — on the seismic behavior of square STC columns. The research is funded by the National Natural Science Foundation of China and provincial science foundations, reflecting its significance in the field of earthquake-resistant structural engineering.

Background and Significance

Steel tube concrete (STC) columns combine the compressive strength of concrete with the ductility and confinement capacity of steel tubes. The square cross-section, while less commonly used than circular sections in large-scale projects, offers advantages in terms of space efficiency, ease of connection to square or rectangular beams, and architectural integration. The seismic performance of square STC columns is of particular interest because:

• The corners of the square section create stress concentration zones under cyclic loading.
• The flat walls are susceptible to local buckling under combined axial and flexural loading.
• The interaction between the steel tube and concrete fill is more complex in square sections than in circular sections due to non-uniform confinement pressure distribution.

Experimental Program

Six full-scale square STC column specimens were designed and tested:

The specimens were subjected to low-cycle reversed loading with displacement control, simulating seismic loading conditions. Strain gauges were applied at critical locations to measure steel tube and concrete strains throughout the test.

Key Experimental Findings

Influence of Steel Ratio

As the steel ratio increases from 6.0% to 12.0%, the following trends are observed:

• Energy dissipation capacity increases — The equivalent viscous damping ratio increases from approximately 0.12 to 0.18, indicating improved seismic energy absorption.
• Ductility improves — The displacement ductility ratio increases, as the additional steel provides greater confinement to the concrete core.
• Stiffness increases — The initial stiffness and post-yield stiffness both increase with steel ratio, but the rate of stiffness degradation after yielding is reduced.
• Failure mode — Higher steel ratios delay local buckling of the steel tube, shifting the failure mode from local buckling to more ductile concrete crushing.

Influence of Axial Compression Ratio

As the axial compression ratio increases from 0.3 to 0.7:

• Horizontal bearing capacity increases — The peak lateral load increases, as the higher axial compression enhances the concrete confinement effect.
• Stiffness decreases — The initial stiffness decreases slightly, and the post-yield stiffness degradation accelerates.
• Energy dissipation capacity decreases — The equivalent viscous damping ratio decreases from approximately 0.15 to 0.10, indicating reduced energy absorption.
• Ductility decreases — The displacement ductility ratio decreases, as the higher axial compression promotes brittle failure modes.
• Failure mode — Higher axial compression ratios lead to more brittle failure, with sudden concrete crushing and steel tube local buckling.

Influence of Slenderness Ratio

As the slenderness ratio increases from 6.0 to 9.0:

• Horizontal bearing capacity decreases — The peak lateral load decreases due to increased second-order effects (P-Δ effects).
• Stiffness decreases — The initial stiffness decreases significantly, as the longer column has greater flexibility.
• Energy dissipation capacity decreases — The equivalent viscous damping ratio decreases, as the column undergoes less plastic deformation before failure.
• Failure mode — Higher slenderness ratios promote overall buckling rather than local buckling, with more pronounced P-Δ effects.

Numerical Simulation Validation

The Abaqus finite element model employs:

• Solid elements (C3D8R) for the concrete core and steel tube.
• Concrete damage plasticity model for the concrete material, calibrated with uniaxial and confined concrete stress-strain curves.
• Von Mises plasticity model with bilinear isotropic hardening for the steel tube.
• Tie constraints between the concrete and steel tube to simulate the composite action.

The numerical results show good agreement with the experimental results:

The slight underestimation of peak load by the numerical model is attributed to the difficulty in accurately modeling the concrete-steel interface behavior under cyclic loading, particularly the progressive loss of bond and the development of interface cracks.

Design Recommendations

Based on the experimental and numerical results, the following design recommendations are proposed for square STC columns in seismic regions:

• Steel ratio — A steel ratio of 8–12% is recommended for seismic applications, as it provides optimal balance between strength, ductility, and energy dissipation.
• Axial compression ratio — The axial compression ratio should be limited to 0.4 or less for seismic design, as higher values significantly reduce ductility and energy dissipation capacity.
• Slenderness ratio — The slenderness ratio should be limited to 8 or less to ensure adequate lateral bearing capacity and prevent overall buckling.
• Steel tube thickness — The steel tube thickness should be designed to prevent local buckling before the concrete reaches its ultimate strain, ensuring ductile failure.
• Corner reinforcement — Local reinforcement at the square section corners may be beneficial to mitigate stress concentration and delay local buckling.

Study Insights

This study provides valuable experimental data on the seismic performance of square STC columns, a structural element that is increasingly used in modern construction due to its architectural and spatial advantages. The systematic investigation of three key parameters — steel ratio, axial compression ratio, and slenderness ratio — provides clear design guidance for engineers. The finding that higher steel ratios improve energy dissipation while higher axial compression ratios reduce it is particularly important for seismic design optimization. The successful validation of the Abaqus numerical model with experimental data establishes a reliable analytical tool for future design studies. The overall conclusion is that square STC columns can achieve satisfactory seismic performance when properly designed, with steel ratios in the 8–12% range, axial compression ratios below 0.4, and slenderness ratios below 8. This work contributes to the growing body of knowledge on composite structural systems and supports the development of performance-based seismic design methodologies for steel tube concrete structures.